The present invention relates to a mobile communication receiver, and is applicable to, for example, portable telephone terminals used in the North American Advanced Mobile Phone Service (hereinafter, AMPS).
AMPS is one example of a mobile communication system in which, when channel conditions between a mobile station (such as a portable telephone set) and its communicating base station (also referred to as a land station) deteriorate, output of the voice signal received by the mobile station is muted.
In the AMPS system, the forward communication channel from a base station to a mobile station occupies a thirty-kilohertz (30-kHz) frequency band. For identification purposes, the base station transmits a supervisory audio tone or SAT at an offset of 5.97 kHz, 6.0 kHz, or 6.03 kHz from the center frequency of the channel. The mobile station is informed of the offset on a separate control channel, and attempts to detect the SAT are regular intervals of, for example, two hundred milliseconds or two hundred fifty milliseconds (200 ms or 250 ms).
A conventional mobile station reproduces the voice signal received from the base station only when the SAT is detected. When the SAT is not detected, the received voice signal is muted until the SAT is detected again, at which time the received voice signal is un-muted. If the SAT is not detected at all for a predetermined period, such as five seconds (5 s), use of the channel is discontinued.
This conventional muting control scheme prevents the user from being distracted by interference and noise when channel conditions deteriorate, and succeeds in maintaining a tolerable level of voice communication quality. In general, the human sense of hearing finds occasional muting easier to tolerate than obtrusive interference or noise.
There are cases, however, in which conventional muting control has a noticeably adverse effect on communication quality. One example is brief fading of the received electric field strength on the channel from the base station to the mobile station, causing the received voice signal to be muted for a single short interval. The resulting unexpected break in an otherwise normal received voice signal is conspicuous and disturbing. Another example is the repeated muting and un-muting that occurs when the received electric field strength hovers around the SAT detection threshold, causing the user to hear a choppy voice signal that is unnatural, hard to understand, and again disturbing.
These problems could be eliminated by dispensing with muting control and using the SAT only to decide when to abandon the channel entirely. Then, however, the user might be even more disturbed by interference and noise occurring during periods of poor channel conditions that are not quite bad enough to force abandonment of the channel.
It is accordingly an object of the present invention to perform muting control in a natural and non-disturbing way in a mobile communication receiver.
The invention provides a method of controlling audio output in a mobile communication receiver that generates an audio signal from a radio signal received on a communication channel. The invented method comprises the following steps:
detecting conditions of the communication channel at predetermined intervals;
muting the audio signal according to a first rule based on the detected channel conditions; and
un-muting the audio signal according to a second rule based on the detected channel conditions, the second rule being less stringent than the first rule.
The invention also provides a mobile communication receiver with a detector for detecting the channel conditions, and a control unit that, by enforcing the first and second rules, makes the transition from muting to un-muting easier than the transition from un-muting to muting.
The asymmetric muting control rule, in which the requirement for muting is more stringent than the requirement for un-muting, prevents sporadic muting and choppy muting, and leads to natural muting control of the audio signal, without disturbing effects.